Euler‘s papers and books are fundamental to every branch of the mathematical sciences. The edition of the series I, II and III is limited, for the most part, to republication of works that Euler himself prepared for press. The texts are reprinted in the original language, most frequently French or Latin. The fourth series (Series IVA) is devoted to Euler’s correspondence; here, along with the texts of the letters in the original language, translations into the “editing language” of each volume have been added.